Fuel Consumption US gal/hr

by Tom Wright in 912 / 914 Technical Questions 11 years ago

I am trying to predict fuel consumption for my Allegro LSA with a Rotax 912 ULS (in the USA).
I am using the Rotax 912 Series Operators Manual, Sep 1, 2012

I get several different predictive answers: 4.9 or 5.3 or 5.9 US gallons/hour.

1 UK gallon = 1.2 US gallon
Assuming the manual talks in UK gallons, I get 5.9 US gal/hr (page 1-14) or 5.3 US gal/hr (page 5-5, when Liters are converted to US gallons)
Assuming the manual talks in US gallons, I get 4.9 US gal/hr (page 1-14) or 5.3 US gal/hr (page 5-5, when 20 Liters are converted to gallons-US)

Can anyone tell me for sure whether the manual uses US or UK gallons and do you think there is an error in the Operators Manual? It seems to disagree with itself regardless of US or UK gallon measures.

Here are my references and logic…..
Page 5-6: The Table indicates 75% = 5000 rpm
Page 1-14, Para 1.10: The Table indicates that at 75% continuous performance, the 912 S/ULS will consume 4.9 gal/hr (UK or US?)
Page 5-5, Figure 5: The figure indicates that at 5000 rpm, the 912 S/ULS will consume 20 L/hr (which equates to 5.3 US gal/hr

Re: Fuel Consumption US gal/hr

by Roger Lee » 11 years ago

Roger Lee

127 5.2k 4.78k

Hi Tom,

The manual is most likely in US gallons. That said there are several variables that affect fuel flow. For example the prop pitch, maybe a fuel transducer K factor difference, poor air flow through the air filter, poor carb sync and aircraft drag. At 5000 rpm it should be between 4.7 to 5.1 depending on the variable factor. At 5000 rpm mine is 4.7. If it were 4.0 on the low side or 5.5 on the high side then you need to take a harder look at what's going on.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Fuel Consumption US gal/hr

by Tom Wright » 11 years ago

Tom Wright

5 6 0

Thanks Roger. Although my post is not really about the "actual" fuel flow for my LSA.

I am trying to ascertain the correct predicted fuel flow for a 912 ULS in U.S. gallons per hour (gph).

If we are to believe the manual is in U.S. gallons, then page 1-14 predicts 4.9 gph at 5000 rpm. But page 5-5 indicates 20 liters per hour at 5000 rpm, which is 5.3 gph.

I'm trying to resolve this discrepancy in predicted fuel consumption.

I want to rely on the Rotax manuals, but have a problem when they don't agree with themselves.

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Re: Fuel Consumption US gal/hr

by Roger Lee » 11 years ago

Roger Lee

127 5.2k 4.78k

Hi Tom,

They may be both correct or at least close. The difference may be in the conversion from Gallons to metric. I would consider both right although 5.3 seems a tad high for only 5000 rpm, but within specs. It is usually below 5 GPH at 5000 rpm. When I travel in my plane I figure 5 GPH at 5150 which seems to bear out with the actual dipping of the fuel tanks when I land, but like my last post stated, there are variable factors that can play into this and can be adjusted to a degree. Prop pitch alone can account for 1 to 1.5 GPH depending on the throttle setting.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

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Re: Fuel Consumption US gal/hr

by James Meade » 11 years ago

James Meade

6 22 2

The table on page 1-14 specifically lists US gph in the upper left label.

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Re: Fuel Consumption US gal/hr

by Tom Wright » 11 years ago

Tom Wright

5 6 0

Thanx much! I was looking all over for that clue ... staring right at it and not seeing. Getting old really sux! That sure answers part of my question.

Now if I can resolve the 20 liters/hour at 5000 rpm on page 5-5 which gives a differrent gal/hr prediction.....

In fact, as best I can tell from flying, I think I get the page 1-14 fuel 4.9 gph consumption rate. But I like to know and trust my manuals.

Thanks again for your good observation.

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Fuel Consumption US gal/hr

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